A technique for improving the quality of a display image through control over the emission luminance of a light source according to a change in the displayed image has been suggested. The technique applies to a liquid crystal display apparatus which includes a liquid crystal panel that modulates light from a light source according to an image signal, and the light source, such as backlight, that illuminates the liquid crystal panel.
For example, a patent document 1 discloses a liquid crystal display apparatus that automatically controls contrast and backlight luminance to adjust them to displayed contents. This liquid crystal display apparatus lowers the contrast of a displayed image when the difference between the maximum luminance level and the minimum luminance level of an input image signal is large, and raises the contrast of the displayed image when the difference is small. The liquid crystal display apparatus also lowers the luminance of the backlight when the average value of the maximum luminance level and minimum luminance level of the input image signal is high, and raises the luminance of the backlight when the average value is low. Thus, the liquid crystal display apparatus always offers constant display luminance.
Besides, the liquid crystal display apparatus allows setting of any desired timer time that is a time span from a point of a change in display contents to a point of execution of control of a contrast value or a backlight luminance value. This prevents the flicker of a screen that happens when display contents is changed in a complicated manner.
A patent document 2 discloses an image display apparatus that remedies an odd feeling in a displayed image resulting from discerning of a change in the luminance of a light source when the emission luminance of the light source is controlled to remedy the deterioration of display quality due to outstanding of a black level of an image. According to this liquid crystal display apparatus, an APL detecting portion detects the APL (Average Picture Level (average luminance level)) of an input image signal to output the detected APL as an APL signal. An intermediate control signal making portion follows a change in the APL of the input image signal, making an intermediate control signal changing for each unit field period. A signal change controlling portion controls a change rate of the intermediate control signal on the basis of a preset time constant, converts the intermediate control signal into a signal having a lower change rate, and puts out the converted signal as a light source control signal.
In this manner, the image display apparatus changes the emission luminance of the light source slowly regardless of a change in the image signal. At this time, a change rate upon controlling the emission luminance of the light source to a lower direction is made higher than a change rate upon controlling the emission luminance to a higher direction. This remedies the deterioration of display quality, such as outstanding of a black tone, due to the higher emission luminance of the light source when an image scene has changed into a dark scene. Meanwhile, unnaturalness due to a change in the luminance of the light source upon a change from a dark scene to a bright scene is also suppressed to improve the quality of a displayed image as a whole.
A patent document 3 discloses a liquid crystal apparatus that remedies the deterioration of the quality of a display image due to a lack of contrast effect and outstanding of a black tone, and that improves the reliability of the apparatus as well. According to this liquid crystal apparatus, an APL detecting portion detects the APL of an input image signal for each unit field period. Based on an APL detection result, a light source control data making portion makes a light source control signal at the minimum level that allows the stable drive of the light source when the APL is from 0% to a given value A1, a light source control signal at the maximum level that allows the stable drive of the light source when the APL is from a given value A2 to 100%, and a light source control signal changing dynamically according to the APL when the APL is from the given value A1 to the given value A2.
The light source is driven based on these light source control signals so that light source control level does not follow a minute change in the APL. This is because that causing the light source control level to follow every minute change of the APL results in a disadvantage of damaging the reliability of the light source, which disadvantage is greater than an advantage of an improvement in contrast, which is not preferable.    Patent document 1: Japanese Laid-Open Patent Publication No. H05-127608    Patent document 2: Japanese Laid-Open Patent Publication No. 2002-357810    Patent document 3: Japanese Laid-Open Patent Publication No. 2003-36063